Wind turbine towers are normally constructed of a number of sections with a cylindrical or conical shape. These sections are prefabricated in a workshop and transported to the place of installation, where they are connected to a wind turbine tower. Typically the sections are connected by bolted joints, comprising nuts and bolts which connect two neighbouring overlapping end sections. The sections are provided with holes, arranged in circumferential direction, through which bolts are inserted in order to form the bolted joints.
The type of connection and the number and size of the required bolts is determined by the magnitude of the expected loads and the number of load cycles with low or high load. For the dimensioning in particular the shear loads are relevant.
Until now typically only prestressed bolts of the “slip resistant at ultimate limit state” type are used for wind turbine towers. Bolts of this kind are appropriate for applications with a low static load capacity and a very high fatigue capacity. During normal operation and when an extreme load occurs loads are transferred as friction by bolts of the slip resistant at ultimate limit state type.
However, when bolts of this type are used, which is denoted as category C in European standard EN 1090, a large number of bolts is needed, which leads to high production costs.